1. Field of the Invention
The present invention relates to a diagnostic imaging apparatus for acquiring image information representing at least one region of an examination subject, which generates image signals that correspond to the acquired image information, of the type having a signal processor, to which the image signals are fed, which subjects the image signals to spatial-frequency filtering with a bandpass characteristic, and which feeds the thus-acquired signals to a display for image representation.
2. Description of the Prior Art
Diagnostic imaging systems of this type are known, for instance which have an X-ray source and an X-ray receiver cooperating therewith that emits image signals corresponding to the received X-rays.
In many applications of such diagnostic imaging systems, for instance in the localization of kidney stones in lithotripsy, it is a problem to reliably distinguish the kidney stones from the background noise. Specifically in adipose patients, the kidney stones are displayed in very poor contrast in the fluoroscopy mode, so that the useful signal, that is, the signal components representing the kidney stone, are significantly below the noise level. This problem is intensified by the fact that the treatment in recidivist cases ensues at an ever earlier time, that is, at a time at which the size of the stones is still very small.
If an unambiguous localization of the kidney stone is not possible, treatment can be very difficult or in certain circumstances impossible.
It may be attempted to improve the signal/noise interval by conventional methods, for instance by integrating several images and subsequently windowing the resultant image. In any case, due to the motion of the stone caused by the patient's breathing, it is not possible to select high integration times, since otherwise the kidney stone would be displayed blurred, or would not be displayed at all because of poor contrast. In addition, windowing is only suitable in cases where homogenous structures are imaged, which is usually not the case in the localization of kidney stones.
The known measure of edge highlighting, which uses spatial-frequency filtering with a high-pass characteristic, usually only makes the signal/noise ratio worse.
Of course, in the case of radiographic diagnostic systems, the image quality can be improved by increasing the X-ray dose; however, this is possible only to a limited degree in view of the radiation load on the patient.
The text "Lexikon der Computergrafik und Bildverarbeitung" (Vieweg Verlagsgesellschaft, 1994, pages 141,142,340 and 341) teaches performing a spatial filtering with a bandpass characteristic, with the result that noise components beyond the upper limit frequency of the bandpass characteristic are suppressed, and subjects of a size exceeding the lower limit frequency of the bandpass characteristic and not visibly displayed or are displayed only in an attenuated manner. Thus, an improvement of the signal/noise ratio is achieved, while an enhancement of the contrast in the display of preferred subjects is achieved as well.
The application of bandpass filters in image processing is also described in German Patent Specification 38 26 285, PCT Application WO 86/01 920, German Patent Specification 195 00 338 and German Patent Specification 41 09 159.